A large number of retroviral oncogenes and their cellular homologues have been identified that play a role in cellular transformation. The product of the Rous sarcoma virus (RSV) oncogene, v-src, is a 60-kDa phosphoprotein, pp60v-src. The cellular oncogene c-src encodes a similar phosphoprotein, pp60c- src. Both proteins possess an intrinsic protein kinase activity that is specific for tyrosine. In addition, both proteins are myristylated at the amino terminus and are associated with the plasma membrane. However, the specific activity of pp60c-src protein kinase is much lower than that of pp60v-src. We are studying the mechanisms leading to the activation of pp60c-src protein kinase and its importance in chemical carcinogen-induced neoplastic transformation of Syrian hamster embryo (SHE) cells. As an initial attempt to study the role of pp60c-src, we compared the level of pp60c-src kinase activity in several chemically induced tumor-derived SHE cell lines and their related preneoplastic cell lines to the level present in normal SHE cells. The pp60c-src activity was measured by autophosphorylation and enolase phosphorylation in immunoprecipitations of cellular extracts containing equal protein formed with a src-specific monoclonal antibody. The fmole 32P incorporated into c- src/min/mg extract protein was determined for each cell line. Tumor-derived cell lines incorporated 4-20 fold more 32P than preneoplastic or normal SHE cells. The increased level of activity was not the result of increased levels of c-src protein as demonstrated by quantitating the amount of (35S) methionine- labelled pp60c-src. These results indicate that the specific activity of pp60c-src is increased in the tumor-derived chemical transformed SHE cell lines. Current research is focused on the mechanism of activation and the effects of the altered c-src on the growth properties of these cells.